Even though returning a value from a validation method is ideal, by n…

…o means is `proposal['value']` the only thing that can be returned (#855)

Co-authored-by: Steven Silvester <steven.silvester@ieee.org>

docs/source/using_traitlets.rst

@@ -122,7 +122,8 @@ Basic Example: Validating the Parity of a Trait
         parity_check.parity = 1
 
 Notice how all of the examples above return
-``proposal['value']``. This is necessary for validation to work
+``proposal['value']``. Returning a value
+is necessary for validation to work
 properly, since the new value of the trait will be the
 return value of the function decorated by ``@validate``. If this
 function does not have any ``return`` statement, then the returned